Preliminary studies suggest that subclinical P. carinii infection is highly prevalent in normal humans and lower animals. Immunosuppression with corticosteroids provokes P. carinii pneumonitis in approximately 100 per cent of conventional Sprague-Dawley rats. Heretofore, a normal, immunocompetent animal model, free of P. carinii, has not been available for epidemiologic studies. We have recently demonstrated that treatment with trimethoprim-sulfamethoxazole renders the animal free of P. carinii. The objective of the proposed research is to determine the mode, or modes, of transmission of P. carinii to normal hosts and to identify a natural habitat of the organism. Basic to our studies is the use of the P. carinii-free normal rat model. Such animals will be exposed to animals with P. carinii pneumonitis, subclinical infection and infected excretions and carcasses to determine if P. carinii can be transmitted via the airborne route and from a simulated non-viable organic habitat in soil and water. The initially P. carinii-free animal, after exposure to infected sources, will be immunosuppressed with corticosteroids to provoke overt P. carinii pneumonitis. In this manner, acquisition of P. carinii by the normal host can be determined. Infection will be identified by histological examination and in-vitro cultivation of lungs for P. carinii and serological studies for antigen and antibody. Once the simulated studies indicate a potential natural habitat, field studies will be done to search for P. carinii in natural specimens.